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Stephen Rainbow (on this page yesterday) seems to make a plausible argument that allowing same-sex marriage will attract back to NZ same-sex couples who want to be married - until we look at the international evidence.

In Australia, studies have found that only about one-fifth of homosexuals and lesbians have shown an interest in same-sex marriage. In the Netherlands, the first country to legally recognise same-sex marriage, studies have shown only around 6 per cent of homosexuals have married during the first five years of legalisation.

In Belgium, it is estimated at less than 5 per cent and in Canada, around 10 per cent on average across the provinces. The US state of Massachusetts seems to be doing the strongest with 16.7 per cent.

Supporters of same-sex marriage argue that civil unions are a second-class marriage but there are many same-sex advocates who argue against "marriage" for same-sex couples, and even suggest that the claim is hurtful to those who have deliberately chosen civil unions.

Marriages are a matter of significant public concern, as the record of almost every culture shows. If it weren't for the fact that sexual intercourse between a man and a woman can lead to children and bring with it a further obligation to care for those children, the notion of marriage would probably never have existed, and the state would not have been interested in it.

Marriage encourages the raising of children by the mother and father who conceived them. On average, children raised by their biological parents who are married have the best outcomes in health, education and income, and by far the lowest involvement with the criminal justice system.

As the prominent Irish homosexual and political commentator Richard Waghorn says, this is certainly not to cast aspersions on other families, but it does underscore the importance of marriage as an institution.

It is true that marriage by definition is discriminatory. A homosexual cannot now legally marry but neither can a lot of other people. A 5-year old boy cannot marry. Three people cannot get married to each other. A married man can't marry another person. Two old aunties living together cannot marry. A father cannot marry his adult daughter. A football team cannot enact group marriage - the list is endless.

It is disingenuous to complain about rights being taken away, when they never existed in the first place.

It is also important to note that marriage is not solely a religious belief. Marriage is a social practice and every culture in every time and place has had some institution that resembles what we know as marriage, associated with procreation. Every society needs natural marriage.

If the law were to allow same-sex marriage, and only same-sex marriage, it could then be argued that we are discriminating against those seeking polygamous, polyamorous, or adult incestuous unions - if all that counts is love and equality.

As then-Labour leader Phil Goff argued at our 2011 Forum on the Family conference, same-sex couples have the option of civil unions to recognise their relationship so there is no need for redefining marriage.

Equality does not mean we must redefine marriage for everyone. Same-sex marriage is, by definition, an oxymoron.

Being pro-marriage and wanting to maintain its definition as being between a man and a woman is not "anti-gay". Gays and lesbians do have a right to form meaningful relationships, they just don't have a right to redefine marriage. The state - which did not invent marriage - has no authority to reinvent it.

Bob McCoskrie is national director of Family First NZ.Debate on this article is now closed.